The siloxane resins of the present invention are frequently designated "MQ" resins. MQ resins are macromolecular polymers consisting essentially of R.sub.3 SiO.sub.1/2 and SiO.sub.4/2 units (the M and Q units, respectively) wherein R is a functional or nonfunctional organic group. Those skilled in the art will appreciate that such resins may also include a limited number of R.sub.2 SiO.sub.2/2 and RSiO.sub.3/2 units, respectively referred to as D and T units. As used herein, the term "MQ resin" means that, on average, no more than about 20 mole percent of the resin molecules are comprised of D and T units.
MQ resins are frequently produced by the acid hydrolysis and subsequent condensation of silicate salts, as disclosed in U.S. Pat. No. 2,676,182 to Daudt et al., Sep. 15, 1950, which is incorporated herein by reference. Such MQ resins are generally produced in such a manner that the resin macromolecules are dissolved in a solvent, which is typically, but not always, an aromatic solvent.
U.S. Pat. No. 2,814,601 to Currie et al., Nov. 26, 1957 discloses that MQ resins can be prepared by converting a water-soluble silicate into a silicic acid monomer or silicic acid oligomer using an acid. When adequate polymerization has been achieved, the resin is end-capped with trimethylchlorosilane to yield the MQ resin.
However, these MQ resins suffer from the drawback that the amount of silanol remaining in the resin after end-capping is large.
U.S. Pat. No. 2,857,356 to Goodwin, Oct. 21, 1958 discloses a method for the preparation of an MQ resin by the cohydolysis of a mixture of an alkyl silicate and a hydrolyzable trialkylsilane organopolysiloxane with water. However, this method suffers from the drawback that large amounts of alkoxy groups and silanol groups remain in the MQ resin, thereby creating the problem of poor storage stability.
U.S. Pat. No. 4,611,042 to Rivers-Ferrell et al., Sep. 9, 1986 discloses a xylene soluble resinous copolymer containing trimethylsiloxane (M) units, alkenyldimethylsiloxane (M) units, and SiO.sub.4/2 (Q) units. The ratio of alkenyldimethoxysilane units to trimethylsiloxane units is 0.02:1 to 0.5:1. The ratio of total M to Q units is 0.6:1 to 1.2:1.
U.S. Pat. No. 4,707,531 to Shirahata, Nov. 17, 1987 discloses a process for preparing an MQ resin by dripping an alkyl silicate into a mixture of aqueous hydrochloric acid and a trialkylsilane or disiloxane at a temperature of 0 to 90.degree. C. This method creates problems from an environmental standpoint because it generates an aqueous hydrochloric acid solution containing large amounts of methanol, and this method is also not capable of preparing high molecular weight Si--H containing MQ resins.
U.S. Pat. No. 4,774,310 to Butler, Sep. 27, 1988 discloses an MQ resin containing silicon-bonded hydrogen atoms and a method for its preparation. The method comprises heating a siloxane resin and a disiloxane of formula (HR.sub.2 Si).sub.2 O in the presence of an acidic catalyst. The reaction mixture formed by heating the resin and disiloxane is then neutralized.
U.S. Pat. No. 5,527,873 to Kobayashi et al., Jun. 18, 1996 discloses a silicone resin and a method for its preparation. The method comprises mixing (a) a silicone resin, (b) an organic solvent, and (c) an acid catalyst to form a reaction product and subsequently reacting the reaction product with a diorganopolysiloxane. The resulting MQ silicone resin has a molecular weight of about 1,000.
Numerous uses have been discovered for MQ resins, including their use as a component in pressure sensitive adhesives, paints, coatings and elastomers. For example, MQ resins can be used in coatings as disclosed in U.S. Pat. No. 3,627,851 to Brady, Dec. 14, 1971, and U.S. Pat. No. 5,034,061 to Maguire et al., Jul. 23, 1991.
MQ resins can also be used in pressure sensitive adhesives as disclosed in U.S. Pat. No. 2,857,356 to Goodwin, Oct. 21, 1958; U.S. Pat. No. 3,528,940 to Modic, Sep. 15, 1970; U.S. Pat. No. 4,831,070 to McInally et al., May 16, 1989; U.S. Pat. No. 4,865,920 No. to Sweet et al., Sep. 12, 1989; U.S. Pat. No. 5,470,923 to Krahnke et al., Nov. 28, 1995; and EP 0 459 292 A2.
Previously, the weight average molecular weight (Mw) of MQ resins has been limited to about 30,000 and the number average molecular weight Mn has been limited to about 7,000. This limitation is inherent in the above-described preparation via hydrolysis and condensation of silicate salts, as attempts to obtain higher molecular weights generally result in an intractable gel.
A higher molecular weight compound based on MQ resins is described in U.S. Pat. No. 4,639,489 to Aizawa et al., Jan. 27, 1987 discloses the reaction of an MQ resin with a hydroxy-terminated polydimethylsiloxane in the preparation of a defoamer. The weight of linear polydimethylsiloxane is, however, much greater than the amount of MQ resin used in the reaction. Thus, the reaction product does not consist essentially of M and Q siloxy units, but consists predominantly of D siloxy units.
Therefore, one object of this invention is to provide an ultra-high molecular weight MQ resin that does not gel. Another object of this invention is to provide an ultra-high molecular weight MQ resin that is soluble in at least one organic solvent in which traditional MQ resins are soluble, such as benzene, toluene, xylene, hexane, heptane, and ketones in order to permit solution reaction of the ultra-high molecular weight MQ resin with other siloxane compounds.